ATLANTA — For decades, it has been one of cancer’s great mysteries: Why do about 4 percent of kidney tumors spontaneously disappear?

Doctors believed that if the immune system was defeating the cancer, treatments to boost it might help the others, but that has not worked very well. Now, three new drugs are displacing the immune system theory and attacking the disease in different ways.

A Pfizer drug, Sutent, prevented tumor growth twice as long as immune therapy did in a study of 750 people whose disease had spread beyond the kidney.

An experimental drug, Wyeth Pharmaceuticals’ temsirolimus, performed even better, boosting survival — not just delaying tumor growth — in a study of 419 very ill patients with widely spread kidney cancer.

Today. fresh results are expected on a third drug — Nexavar, made by Bayer and Onyx Pharmaceuticals Inc.

“Until just a few years ago, there were no promising drugs for kidney cancer,” said Dr. Gary Hudes of Fox Chase Cancer Center in Philadelphia, who led the study on the Wyeth drug.

There still is no cure, “but these drugs can control the disease for a significant amount of time,” and may offer more benefit when given earlier in the course of the disease, he said.

The drugs were discussed Sunday at a meeting in Atlanta of the American Society of Clinical Oncology.

About 39,000 new cases of kidney cancer and 12,800 deaths from it are expected in the United States this year. Smoking is the top risk factor. About one-third of cases spread throughout the body.

Temsirolimus is part of a new generation of cancer drugs that, unlike chemotherapy, attack cancer in more precise ways. This drug is the first to target mTOR, a signaling chemical that controls cell growth and the formation of blood vessels that nourish tumors.

Patients who received the drug lived about 11 months versus about seven months for those given interferon, an immune system treatment.

Fatigue and severe side effects were more common in those receiving interferon. Mild anemia, rash and mouth sores were more common with the novel drug “but easily manageable,” Hudes said.

“This is the first trial that really validates mTOR as a cancer target,” Hudes said.

In the other study, Sutent, which targets different cell signals, delayed the growth of cancer for 11 months compared to five months for those given interferon, said Dr. Robert Motzer of Memorial Sloan-Kettering Cancer Center in New York.

High blood pressure, diarrhea and fatigue occurred in 5 percent to 8 percent on the new drug — acceptable side effects given the seriousness of the cancer, doctors said.

The drugs’ manufacturers paid for the studies and many of the researchers have consulted for the companies.

“Those results look really good,” said Dr. Len Lichtenfeld, deputy chief medical officer of the American Cancer Society. It has been “a watershed year” for finding new ways to treat the disease, he said.

Doctors and patients now have the happy dilemma of deciding which drug to try. Nexavar came on the market in December and Sutent, in January. Wyeth plans to seek approval to sell temsirolimus by the end of this year.

In other news at the conference, researchers reported that a novel vaccine against cervical cancer, expected to receive federal approval this week, appears to have an added bonus: preventing two other gynecological cancers.

The vaccine, Gardasil, is designed to protect against four types of human papillomavirus (HPV), including the two believed responsible for about 70 percent of cervical cancer cases.

Combined results from three studies involving 18,150 women around the world found that the vaccine also completely prevented vaginal and vulvar cancers linked to those four types of HPV.

With about two years of followup, no cases developed in women who received three doses of Gardasil, but 24 cases occurred in women who received dummy vaccine.

“It adds to the health benefits we get from this vaccine,” said Dr. Jorma Paavonen of the University of Helsinki in Finland, who led the analysis.

“This is great news,” said Dr. Carolyn Runowicz, the cancer society’s president. The society estimates that 6,000 new vaginal and vulvar cancers will occur in the United States this year, and about 1,700 women will die from them.

People with head and neck cancers also got some good news. Adding the Sanofi-Aventis drug Taxotere to standard chemotherapy and radiation treatment resulted in 62 percent survival at three years versus 48 percent for those who received only the standard therapy, said Dr. Marshall Posner, Dana-Farber Cancer Institute and Harvard Medical School.

“This is going to be a practice-changing,” said Dr. Sandra Horning, a Stanford University doctor who is president of the oncology society.

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